1. Technical Field
The present invention relates in general to hybrid drill bits and, in particular, to an improved system, method, and apparatus for a hybrid drill bit having a combination of rolling cones and fixed cutter elements for cutting at a center of the drill bit.
2. Description of the Related Art
In the prior art, some drilling bits use a combination of one or more roller cones and one or more fixed blades. Some of these combination-type drill bits are referred to as hybrid drill bits. Previous designs of hybrid drill bits, such as U.S. Pat. No. 4,343,371, to Baker, III, have provided for the roller cones to do most of the formation cutting, especially in the center of the hole or bit. Other types of combination drill bits are known as “core bits,” such as U.S. Pat. No. 4,006,788, to Garner. Core bits typically have truncated roller cones that do not extend to the center of the bit and are designed to remove a core sample of formation by drilling down but around a solid cylinder of the formation before being removed.
Another type of hybrid drill bit is described in U.S. Pat. No. 5,695,019, to Shamburger, Jr., wherein the roller cones extend almost entirely to the center. Fixed cutter inserts 50 (FIGS. 2 and 3) are located in the dome area 2 or “crotch” of the bit to complete the removal of the drilled formation. Still another type of hybrid bit is sometimes referred to as a “hole opener,” an example of which is described in U.S. Pat. No. 6,527,066. A hole opener has a fixed threaded protuberance that extends axially beyond the roller cones for the attachment of a pilot bit that can be a roller cone or fixed cutter bit. In these latter two cases the center is cut with fixed cutter elements but the fixed cutter elements do not form a continuous, uninterrupted cutting profile from the center to the perimeter of the bit.
Although each of these drill bits is workable for certain limited applications, an improved hybrid drill bit with enhanced drilling performance would be desirable.